spi : spi-topcliff-pch: Fix to handle empty DMA buffers

pch_alloc_dma_buf allocated tx, rx DMA buffers which can fail. Further,
these buffers are used without a check. The patch checks for these
failures and sends the error upstream.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Mark Brown <broonie@kernel.org>